determining wedge or dash in alkene stereospecific reaction

[orangepopsicle]

Hey guys,

I'm having a really hard time determining how these people determined what was a wedge or dash in the alkene before the reaction happened and after the reaction happened. If you see the image below, it shows the alkene substituents in various wedge or dash notation after the reaction has happened but I have no idea how they made that determination, can anyone help me out?

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[510586]

when chlorine adds to the double bond it adds cis (or syn), and all the other Atoms stay in their original confirmation. Since it is cis, cl are on the same side. If the methyl group is on one side and the opposite side is also on that side it will remain facing in the same direction - but you don't need to know which one is wedge which is dash.

 

[orangepopsicle]

Then how can I know which of these compounds will end up as a meso compound when reacted with Cl2,CCl4? This was question 16 on DAT Bootcamp Orgo Test 5. Without knowing the wedge dash configuration of the alkene substitutents, we would never be able to make a determination which one will end up as a meso compound...

 

[510586]

Then how can I know which of these compounds will end up as a meso compound when reacted with Cl2,CCl4? This was question 16 on DAT Bootcamp Orgo Test 5. Without knowing the wedge dash configuration of the alkene substitutents, we would never be able to make a determination which one will end up as a meso compound...

Do R and S configurations . if they are both chirality centers and have plane of symmetry (R one side S other) then they are meso. Like cis-1,2-dichlirocyclohexane for example. Example c is S and S or R and R so it is not meso it is enantiomer

 

[teefdocta]

Would halogenation of Alkenes are always Anti though? And this is my though for rxn#2. They did go under rotation to have both Cl on the same side

 

[DAT Destroyer]

Hey guys,

I'm having a really hard time determining how these people determined what was a wedge or dash in the alkene before the reaction happened and after the reaction happened. If you see the image below, it shows the alkene substituents in various wedge or dash notation after the reaction has happened but I have no idea how they made that determination, can anyone help me out?

Hello Runswithamango

It is confusing indeed. A much better way to do this is as follows..... Let us consider adding chlorine to cis- 2-butene. .If you add halogens across a double bond......put them in the anti position. Now.....since the methyls were cis ......keep them cis......draw the new product as either a PAIR of wedges on the methyls or a pair of dashes. You now should have chlorines anti....methyls as both dashes or wedges..... If you now examine the stereochemistry ....one pair is RR and the other is SS. Now let us do trans-2-butene. Break the double bond.....add the 2 chlorines anti...... since we started with trans.....keep one methyl as a dash...the other as a wedge. If done correctly.....you would find this is RS or SR.......a meso compound. Thus...no enantiomer If you had trouble following this....I have drawn it out for you......

Hope this helps.

Dr. Jim Romano

 

[jsbio]

Thank you Dr. Romano your post above just made it crystal clear.

 

[DAT Destroyer]

Thank you Dr. Romano your post above just made it crystal clear.

Glad I could help it is not the easiest topic..Keep up the good work and hammering away..

Dr. Romano

 

[OSUkid]

Hey guys,

I'm having a really hard time determining how these people determined what was a wedge or dash in the alkene before the reaction happened and after the reaction happened. If you see the image below, it shows the alkene substituents in various wedge or dash notation after the reaction has happened but I have no idea how they made that determination, can anyone help me out?

I know what your asking, here is the answer:

The Carbons are sp2 hybridized, or vinylic carbons in the alkene, yes? (Double bonded carbons are sp2) So it is trigonal planar, so all four groups attached to the vinylic carbons are in the same plane, so thats why you can write the bottom substituents as wedged and the top as dashed. You do this to better understand if it is anti or syn addition. So if you ever come across syn or addition questions just write the bottom as wedged and top as dashed to better understand where the reagents are going and what is the stereochemistry.

 

[orangepopsicle]

thanks guys, this really helped a lot!